Despite great progress in the automation of the cockpit, the appropriate management of the energy of the aircraft during the last stages of a flight is still a difficult task.
A flight management and guidance system, of the FMGS (Flight Management and Guidance System) type, is capable of making the aircraft fly along a predefined trajectory—comprising appropriate energy management—with minimum intervention by the pilot.
Although potentially very efficient, this solution is not the most frequently used one in real operations. In fact, air traffic control often asks aircraft which are approaching an airport follow radar guidance well before a final approach point, until being authorized to carry out the final approach.
Radar guidance provides more flexibility to air traffic control for managing unexpected situations which can, for example, be caused by unstable weather or by risks of conflict with other traffic.
Moreover, once the aircraft has abandoned a pre-planned trajectory for radar guidance, no complete trajectory (from the aircraft to the runway) is then defined. The FMGS system cannot therefore compute the guidance and control commands.
In such a situation, the most convenient way for the crew to fly the aircraft is based on the use of the automatic pilot system AP (comprising an “autothrottle” or “autothrust” AT if available).
This system allows the crew to define altitude, heading and speed targets directly via the man/machine interface of the AP/AT system. Regarding the way in which an altitude target is attained, this is normally carried out either by entering a vertical speed or slope target or by asking the FMGS system to adjust the thrust (for a constant airspeed) in order to make the aircraft climb or descend. Once the targets have been entered, the AP/AT system computes the control commands which are intended for the appropriate sub-systems (for example the engines, the control surfaces, etc.) in order to attain and follow these targets.
However, the AP/AT system does not help the crew to manage the total energy state of the aircraft, except for the reduction of workload derived from the automated following of the components of the target vector.
From a flight mechanics point of view, the authorization or the clearance (“clearance” being the English term) given by air traffic control, at the end of each loop of negotiation with the crew, can be considered as a new target total energy state of the aircraft. In particular, the speed clearance represents a target kinetic energy state, whereas the altitude clearance represents a target potential energy state. The heading clearances have no direct effect on the target total energy, but they define a lateral path and thus the total distance that the aircraft will have to fly before reaching the threshold of the runway.
A certain number of solutions have been proposed in order to solve this problem and, in certain cases, have been installed in avionics systems in order to provide the crew with assistance in the management of the energy in radar-guided operations.
These solutions comprise graphical symbols, generally displayed on the navigation display ND (“Navigation Display” being the English term) which provide the crew with visual indications for a better evaluation of the energy state.
However, the crew must still manage the energy of the aircraft. It always has the complete task of monitoring the energy state of the aircraft and of modifying, if necessary, the way in which the aircraft flies towards the prescribed targets (that is to say by acting on the airbrakes or by changing the vertical speed value programmed in the window of the automatic pilot system).